THE NORTH POLAR BASIN. 385 



animals? How many genera of molhisks are equal to a genus of 

 mammals, or how many butterflies are equal to a bird? 



If there be any region of the world with any claim to be a life area, 

 it is that part of the Polar Basin which lies between the July isotherm 

 of 50° or 530 F. and the northern limit of organic life. The former 

 corresponds very nearly with the northern limit of forest growth, and 

 they comprise between them the barren grounds of America and the 

 tundras of Arctic Europe and Siberia. 



The fauna and flora of this circuinpolar belt is practically homo- 

 geneous; many species of both plants and animals range throughout 

 its whole extent. It constitutes a circumpolar Arctic region, and can 

 not consistently be separated at Bering Strait into two parts of suliQ- 

 cient importance to rank even as sub-regions. 



Animals recognize facts and are governed by them in the extension 

 of their ranges; they care little or nothing about generalizations. The 

 mean temperature of a province is a matter of indiflerence to some 

 plants and to most animals. The facts which govern their distribution 

 are various, and vary according to the needs of the plant or animal 

 concerned. To a migratory bird the mean annual temperature is a 

 matter of supreme indifference. To a resident bird the question is 

 equally beside the mark. The facts which govern the geographical 

 distribution of birds are the extremes of temperature, not the means. 

 Arctic birds are nearly all migratory. Their distribution during the 

 breeding season depends primarily on the temperature of July, which 

 must range between 53° and 35° F. It is very important however to 

 remember that it is actual temperature that governs them, not iso- 

 therms corrected to sea level. If an Arctic bird can find a correct iso- 

 therm below the Arctic Circle by ascending to an elevation of 5, 000 or 

 G,000 feet above the level of the sea, it avails itself of the opportunity. 

 Thus the region of the Dovrefeld above the limit of forest growth is 

 the breeding place of many absolutely Arctic birds; but this is not 

 nearly so much the case on the Alps, because the cold nights vary too 

 much from the hot days to come within the range of the birds' breed- 

 ing grounds. Here, again, the mean daily temperature is of no impcn- 

 tance. It is the extreme of cold which is the most potent factor in this 

 case, and no extreme of heat can counter-balance its eft'ect. 



POLAR ISOTHERMALS. 



In estimating the influence of elevation upon temperature it has been 

 ascertained that it is necessary to deduct about 3*^ F. for every thou- 

 sand feet. The isothermal lines are very eccentric in the Polar Basin. 

 The mean temperature of summer is quite independent of that of 

 winter. The isothermal lines of July are regulated by geographical 

 causes which do not affect those of December or operate in a contrary 

 direction. The Gulf Stream raises the mean temperature of Iceland 

 during winter to the highest point which it reaches jii the Polar Basin, 

 gjM 93 25 



